Multi-speed power transmissions, particularly those using planetary gear arrangements, require a hydraulic system to provide controlled engagement and disengagement, on a desired schedule, of the clutches and brakes or torque transmitting mechanisms that operate to establish the ratios within the planetary gear arrangement.
These control systems have evolved from substantially pure hydraulic control systems, wherein all of the control signals are produced by hydraulic devices, to electro-hydraulic control systems, wherein a number of the control signals are produced by an electronic control unit. The electronic control unit emits electrical control signals to solenoid valves, which then issue controlled hydraulic signals to the various operating valves within the transmission control.
With many of the early pure hydraulic and first generation electro-hydraulic control systems, the power transmission utilized a number of freewheel or one-way devices which smooth the shifting or ratio interchange of the transmission during both upshifting and downshifting of the transmission. This relieves the hydraulic control system from providing for the control of overlap between the torque transmitting mechanism that was coming on and the torque transmitting mechanism that was going off. If this overlap is excessive, the driver feels a shudder in the drivetrain, and if the overlap is too little, the driver experiences engine flare or a sense of coasting. The freewheel device prevents this feeling by quickly engaging when the torque imposed thereon is reversed from a freewheeling state to a transmitting state.
The advent of electro-hydraulic devices gave rise to what is known as clutch-to-clutch shift arrangements to reduce the complexity of the transmission and the control. These electro-hydraulic control mechanisms are generally perceived to reduce cost and reduce the space required for the control mechanism.
In addition, with the advent of more sophisticated control mechanisms, the power transmissions have advanced from two-speed or three-speed transmissions to five-speed and six-speed transmissions. In at least one presently available six-speed transmission, just five friction devices are employed to provide six forward speeds, neutral condition, and a reverse speed. Such a gear arrangement is shown in U.S. Pat. No. 4,070,927 issued to Polak on Jan. 31, 1978. The use of the planetary gearset shown in the Polak patent has given rise to a number of electro-hydraulic control mechanisms, such as that shown in U.S. Pat. No. 5,601,506, issued to Long et al. on Feb. 11, 1997. The torque capacity of a torque transmitting mechanism (on-coming or off-going) involved in a shift may be conveniently controlled by the combination of an electrically activated solenoid valve and a pressure regulator valve or trim valve, as disclosed, for example, in the U.S. Pat. No. 5,911,244 to Long et al., issued on Jun. 15, 1999, assigned to the assignee of the present invention, and incorporated herein by reference. In a typical system, the solenoid valve is activated by pulse-width-modulation (PWM) at a controlled duty cycle to develop a pilot pressure for the pressure regulator valve or trim valve, which in turn, supplies fluid pressure to the torque transmitting mechanisms in proportion to the solenoid duty cycle.
Additionally, an electrically variable hybrid transmission has been proposed to improve fuel economy and reduce exhaust emissions. The electrically variable hybrid transmission splits mechanical power between an input shaft and an output shaft into a mechanical power path and an electrical power path by means of differential gearing. The mechanical power path may include clutches and additional gears. The electrical power path may employ two electrical power units, or motor/generator assemblies, each of which may operate as a motor or a generator. With an electrical storage system, such as a battery, the electrically variable hybrid transmission can be incorporated into a propulsion system for a hybrid electric vehicle. The operation of such an electrically variable hybrid transmission is described in the U.S. Pat. No. 6,551,208 to Holmes et al., issued on Apr. 22, 2003.
The hybrid propulsion system uses an electrical power source as well as an engine power source. The electrical power source is connected with the motor/generator units through an electronic control unit, which distributes the electrical power as required. The electronic control unit also has connections with the engine and vehicle to determine the operating characteristics, or operating demand, so that the motor/generator assemblies are operated properly as either a motor or a generator. When operating as a generator, the motor/generator assembly accepts power from either the vehicle or the engine and stores power in the battery, or provides that power to operate another electrical device or another motor/generator assembly.
It is important to reliably and inexpensively diagnose torque transmitting mechanism engagement and disengagement in the above described torque transmitting mechanism controls, both to verify shift progression, and to detect an inadvertent engagement or disengagement. This can be accomplished either indirectly by analyzing the transmission input and output speeds, or directly by installing pressure switches at each of the clutches. However, the diagnostic output with either of these techniques is only developed once the respective clutch has actually started to engage or disengage, which is not especially desirable if the engagement or disengagement is inadvertent. Theoretically, one could alternatively measure the pilot pressure or the position of the trim valve, but such approaches may be expensive to implement and inaccurate due to the characteristic dithering of a trim valve.
An effective way to determine a change in the trim valve position is to provide a pressure sensitive switch in fluid communication with the trim valve and operable to be selectively pressurized or de-pressurized when the trim valve changes position. Traditionally, this method of valve state diagnostics would require a separate pressure switch for each valve, i.e. six pressure switches for six valves. However, packaging space and cost constraints may make this option unfeasible. Accordingly, what is needed is an inexpensive clutch pressure control arrangement that provides a reliable diagnostic output early in the clutch pressure control process while relying on a minimum of pressure switches.